Within the group of reactive adhesives the methacrylate systems are becoming increasingly important. They have been commercially available for years as a two component formulation. Component A (adhesive component) usually exists as a solution of elastomers in a monomer or a monomer mixture based on acrylic acid and/or especially methacrylic acid. Component B (activator or hardener component) contains an initiator to initiate free radical polymerization.
Both components prior to their use are thoroughly mixed together, thereby starting the polymerization of the monomers.
A further development of the so-called two component systems are the "No-Mix" or "Second Generation Acrylate adhesives". The use of these adhesives is similar to that of the contact adhesives, i.e. the initiator component is first applied as a thin layer on one or both of the surfaces to be glued together. After a waiting period, which can be up to several hours, the adhesive component is then applied. The bonding is carried out by fixing the components in the desired position.
In the adhesive component polymerizable monomers can be used alone or in combination, especially methacrylic acid or a methacrylic acid derivative such as methyl methacrylate, or suitable acrylic acid compounds, e.g. butyl acrylate. In addition to these monomers, other systems were investigated, which to some extent have gained a measure of technical importance. Examples of these are: ethylene glycol dimethacrylate and oligoethylene glycol dimethacrylates, (meth)-acrylic ester of tricyclo[5.2.1.0.sup.2,6 ] decane, glycerine (meth)acrylate, bisphenol-A-derivatives, perfluoborated dimethacrylates, aromatic diester-dimethacrylates, aromatic diether dimethacrylates and aromatic ester ether dimethacrylates, urethane dimethacrylates, diurea dimethacrylates, and other compounds. From the vast relevant literature, the following are mentioned as examples thereof: Polymer Science and Technology 14, 357, 373 and 379 (1981); J. Dent. Res. 58, 1544 and 1981 (1979), J. Dent. Res. 58, Abstr. No. 1216, 395 (1979), J. Dent. Res. 49, 810 (1970) and 52, 731 as well as 1128 (1973), U.S. Pat. Nos. 4,131,729, 3,066,112, 3,179,623, 3,810,938 as well as German Application Nos. 2,432,013, 2,312,559 and 2,411,760. In order to improve cohesion, to adjust to suitable viscosity for application purposes, and to reduce the volume contraction during hardening, polymers are mixed with the monomers in the adhesive component. Examples thereof are polymethylmethacrylates, polychloroprene, chlorosulfonated polyethylene, nitrile rubbers and/or polyurethanes.
The classical hardeners for room temperature polymerizable methacrylate systems consist of benzoyl peroxide as the initiator and tertiary aromatic amines, especially N,N-diemthyltoluidine, as the accelerator.
Today there is a broad pallet of accelerators at our disposal. Also of importance are, in this connection, new hardener systems based on organoboron compounds, which when exposed to oxygen in the air will initiate the polymerization reaction, thereby making them particularly suitable for work at room temperature. New hardening systems based on such organoboron compounds are the subject matter of a number of prior patent applications of the applicant.
Reaction adhesives of the organoboron type exhibit a number of useful properties. For example, they exhibit a broad spectrum of applications for various surfaces with little requirements for pretreatment of the surfaces, they harden quickly if desired even at room temperature, and through the proper selection of reactive monomers or monomer mixtures they can be adjusted to give good flexibility values for the adhesive layer. High stress shearing strengths can be obtained. When working with these adhesives, the so-called open time can be regulated. The adhesive bond will exhibit less shrinkage and good reproducibility, while exhibiting good resistance to solvents and temperatures.
The dominant constituents of the adhesive components are still mono(meth)-acrylates, which exhibit high vapor pressure. However, the use thereof not only causes an offensive odor, but consequently the adhesive component, for example for No-Mix applications, can only be stored open for a short while.
Bismethacrylates normally have higher boiling points. A number of these types, especially bismethacrylates with high molecular weight, are normally solid and therefore can be used as monomers in adhesives to only a minor extent. Furthermore, with respect to bismethacrylates with low molecular weights, the distance between the functional groups is small and the corresponding polymers are highly cross linked and have a tendency to become brittle.